The use of ultraviolet absorbers for the protection of polymeric compositions is well-known. A review of the various product types is provided by J. E. Pickett, Handbook of Polymer Degradation, 2nd Ed., S. Halim Hamid, ed. Marcel Dekker, NY, pp 163–190, (2000). The common structural types are: Benzophenones, Benzotriazoles, Triazines, Oxanilides, Cyanoacrylates, Malonates, Formamidines and Benzoxazinones.
These products are generally designed to absorb solar ultraviolet radiation in the 290–390 nm wavelength range, with maximum absorption in the regions where the commercial polymers exhibit their greatest sensitivity. For example, the greatest sensitivity to yellowing of various polymers is as follows: polycarbonates (295 and 310 –340 nm), polystyrene (319 nm), polyethylene (310 nm), ABS (330 nm), and PVC (308–325 nm): N. D. Searle, Handbook of Polymer Degradation, p. 615. Since most polymers have their greatest sensitivity at wavelengths below 340 nm, or the UVB region, the ultraviolet absorbers will generally show weaker absorption in the wavelengths above 340 nm, or the UVA region of the solar spectrum. An additional reason for the relatively short wavelength absorption of the commercial ultraviolet stabilizers is to prevent their absorption from extending into the visible region greater than 400nm, thereby imparting color to the polymeric materials in which they are incorporated.
If ultraviolet absorbers are used alone for ultraviolet screening applications, they will show deficient light absorption in the UVA region, to which many foods are sensitive, and if they are used at sufficiently high concentrations to provide strong absorption in the UVA region, they will impart color to the polymers in which they are incorporated.
Optical brighteners (also known as fluorescent whitening agents and optical bleaches) have been used for many years to impart a desirable “blueness” to polymers. These are compounds which absorb ultraviolet radiation in the ultraviolet region and re-emit it as fluorescence in the blue region of the visible spectrum. Detailed reviews of optical brightener structures, properties and end-uses have been provided in: Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., Interscience, NY, 4, 213–220 (1978); and J. T. Lutz, Jr., Thermoplastic Polymer Additives, Marcel Dekker, NY, pp 315–343 (1989).
This invention provides polymeric compositions which overcome the deficiencies of the ultraviolet absorbers alone as screening agents, and which provide excellent screening protection throughout the solar ultraviolet region without adding undue color to the polymeric compositions.